1. Field of the Invention
The present invention relates generally to navigational systems and, in particular, to electronic, portable navigational systems that use radionavigational data and dead reckoning for foot navigation.
2. Description of Related Art
The term "dead reckoning" (DR) refers to a position solution that is obtained by measuring or deducing displacements from a known starting point in accordance with motion of the user. Two types of DR are known: inertial navigation and compass/speedometer.
Inertial Navigation Systems (INS) use data from three orthogonal accelerometers. Double integration calculates position from acceleration as the user moves. Three gyros are also required to measure the attitude of the accelerometers and remove the effects of gravity. Results of the integration are added to the starting position to obtain current location. The need for six accurate and stable sensors makes the cost of INS high.
INS position errors increase with the square of time due to the double integration. These errors could easily be a large fraction of a foot traveler's ground speed. A typical INS system has a drift rate on the order of 0.8 miles per hour (mph). Although this error could easily exceed the rate of travel for a person walking on foot, it may be negligible for a jet aircraft. Future developments in inertial sensors may lower costs, but improvements in accuracy are less predictable. For these reasons, inertial navigation may not be a viable option for low-cost foot traveler navigation.
Regarding compass/speedometer DR systems, a compass and a speed/distance sensor is a direct means to determine location, and has been automated with microcomputers in vehicular applications. Simple and low-cost DR systems for land navigation have often been built using only a compass and a ground speed sensor for measurements.
Generally, these systems have been designed for automobiles or trucks, with the speed sensor output being derived from rotation of the road wheels. Strap-down inertial navigation systems have become popular due to the numerical processing capability of microprocessors and are lower cost and power than stable platform inertial systems.
A considerable amount of work has been done related to the integration of a Global Positioning System (GPS) and INS. The importance of supplementing the GPS position solution with an integrated GPS/DR system is evidenced by the considerable work that has been done in this area. Work has not been done specifically for foot traveler, hiker, or pedestrian use in this area.